Optical bar code scanners are being used for a number of different purposes. The best known application for such scanners is in check-out operations at supermarkets or other retail stores. In this application, the scanner detects a bar code printed on or attached to a product and uses the decoded information to retrieve the identity and current price of the product from a system memory. The product identity and price are used primarily to prepare customer receipts. The same information may also be used for other purposes. For example, the product identity may be used in an inventory control system to track current stocks of a particular product and to automatically reorder the product when the stock falls below a predetermined level.
Optical bar code scanners are also used in industrial and/or manufacturing environments. One use of a bar code scanner in such environments is to identify incoming or outgoing labeled materials to control the routing of materials through an automated conveyor system. Another use is to track labeled parts or sub-assemblies on an assembly line to assure that the proper parts and sub-assemblies are available when and where they are needed for final assembly of a complex end product, such as an automobile.
Because there are fundamental differences between the requirements of retail and industrial/manufacturing environments, the same type of optical bar code scanner is not necessarily suitable for use in both kinds of environments. In the retail environment, the product carrying the bar code label can usually be brought to the scanner by the check-out stand operator. Therefore, it is not generally considered critical that a retail check-out scanner be capable of reading bar code labels at widely varying distances from the scanner.
In an industrial/manufacturing environment, on the other hand, it is not always possible for a product to be manipulated so as to bring the bar code label within a limited range of distances from a scanner. In such an environment, the item carrying the bar code label may be too heavy or too bulky to allow the item to be repositioned. In some highly automated systems scanners operate in an unattended mode, which means simply that there is no human operator for the scanner. In such systems, an operator is not available to reposition an item for scanning even where it might be physically feasible to do so.
Different techniques have been adopted in attempts to solve problems encountered in reading bar code labels in industrial/manufacturing environments. Where the label can't be brought to the scanner, the simplest approach is to bring the scanner to the label by using a hand-held or portable scanner. One problem with this approach is that a label may not be located in an easily accessible spot on the item being tracked. Another problem is that an operator must always be available to perform what is basically a mechanical function; namely, maneuvering the hand-held scanner into a position in which the label can be read. The presence of an operator adds to the cost of any operation being performed. Even if costs were not a factor, it may not always be possible to use a human operator because the environment in which the operation is being performed is hostile to humans; e.g., conditions of extreme heat or extreme cold or the existence of toxic vapors.
Because hand-held scanners are not well suited for certain industrial/manufacturing applications, attempts have been made to develop fixed position scanners for such applications. Because the distance between the label and the scanner may vary widely, such fixed position scanners must be designed with a large depth of field. The "depth of field" of the scanner is the range of distances over which the scanner can read the smallest bar code label allowed by the standards authority for the particular bar code being read. For example, the Uniform Product Code Council has issued detailed specifications requiring that UPC (Universal Product Code) labels be no smaller than a predetermined minimum size.
Known scanners employ rotating beam deflectors capable of generating multiple scan lines having different focal lengths; that is, focussed at points at different distances from the scanner. Such scanners can provide a set of scan lines (a scan line pattern) that is capable of reading even small labels at varying distances. However, to achieve an acceptable reading performance over a range of distances, it may be necessary to adjust certain parameters of scanner operation as a function of the characteristics of the different scan lines. For example, a label at a considerable distance from a scanner may be read by a scan line having a long focal length. Since the strength of a returned optical signal is inversely proportional to the distance between the label and the scanner, it may be necessary to increase the gain of the scanner circuits when reading such a label.
A number of different approaches have been suggested for deriving information needed to control the operation of a scanner on a per scan line or per facet basis. Japanese patent application No. 57-116558 discloses a holographic scanner having a rotating transparent substrate with transmission holograms on the lower surface. A laser beam directed toward the lower surface of the substrate is deflected by the transmission holograms. While most of the deflected light exits from the top surface of the substrate, a small portion is internally reflected back toward the underside of the disk. The internally reflected light is detected by a photodetector. The photodetector output is compared with a prescribed value to provide a correction signal which is used to provide uniform intensity of the scanning beam.
One drawback to this apparatus is that the auxiliary photodetector must be located along the path of the internally reflected beam, which is fixed by the physical/optical geometry of the scanner. It may be difficult to locate a photodetector within the scanner housing along this fixed path.
Japanese patent application No. 57-109196 discloses an optical scanner which includes an auxiliary photodetector for detecting laser light reflected from the lower surface of a rotating substrate. The detected light is identified as zero-dimensional (or zero order) reflected light. The output of the auxiliary photodetector is used to control a modulator which regulates the intensity of the laser beam impinging on the substrate. Since the path of the zero-order beam is fixed in accordance with well known laws of optics, the auxiliary photodetector must be located along this fixed path. As mentioned above, it may be difficult as a practical matter of scanner design to locate the auxiliary photodetector on the required path.
U.S. Pat. No. 4,548,463 discloses an optical scanner in which an auxiliary photodetector is located on the opposite side of the holographic disk from the laser light source. While most of the light is deflected by transmission holograms on the holographic disk, a portion of the light (the zero-order beam) continues along the original beam path to the auxiliary photodetector. The output of the photodetector can be used to control video amplifier gain or semiconductor laser current to control laser beam output.
Since the path of the transmitted zero-order beam is fixed, the auxiliary photodetector must be located at a point along that path. Because of space constraints, it may be difficult to locate an auxiliary photodetector on the opposite side of the holographic disk from the laser light source.
An IBM Technical Disclosure Bulletin (Vol. 25, No. 3B, Aug. 1982, page 1599) suggests that an auxiliary data track be encoded at the rim of a holographic disk. Optically or magnetically encoded data in this auxiliary data track could be used to set scanner gain, among other things. While the approach suggested in the article would make it possible to vary scanner parameters on a per facet basis, there are drawbacks to the approach. Means must be provided to both record and read the data track. These means represent added cost. Also, it may be difficult to locate the necessary sensor adjacent the periphery of the disk within a scanner housing.